Moire is a wavy, often colored interference pattern that appears in digital photographs when a repeating pattern in the subject (fine fabric weaves, brick walls, distant fences, herringbone suits, fine printed text, computer screens) has a spatial frequency close to the pixel pitch of the sensor. The sensor samples the scene at discrete intervals, and when the subject’s frequency aligns with that grid in a near-resonance, false patterns and rainbow colors emerge that do not exist in the original scene.
The phenomenon is mathematically related to aliasing in signal processing. The Nyquist sampling theorem says that to faithfully record a frequency, you must sample at least twice as fast. When a subject contains detail finer than half the sensor’s pixel pitch, the sensor cannot represent it correctly, and the misrepresented signal aliases into lower frequencies that show as moire. The colored fringes come from the Bayer pattern: each color channel samples at a slightly different effective frequency, so the aliasing produces different errors per channel.
Cameras historically combated moire with an anti-aliasing or optical low-pass filter (the AA filter), a thin birefringent layer in front of the sensor that slightly blurs incoming light to suppress frequencies above the Nyquist limit. The tradeoff was a small loss of perceived sharpness. As resolution increased to 36, 45, 60, and 100 megapixels, manufacturers began removing the AA filter (Nikon D800E, D850; Sony A7R series; Fujifilm GFX 100) on the reasoning that the pixel pitch had become small enough that moire was rare in practice and the sharpness gain was worth it.
In the field, several techniques mitigate moire when it appears. Slightly defocusing softens the offending frequency to below the threshold. Shifting the camera angle (a few degrees of rotation) changes how the subject’s grid aligns with the sensor’s grid and often eliminates the pattern. Stepping closer or further changes the projected frequency on the sensor. Stopping down enough to invoke diffraction can also reduce moire, though at the cost of overall sharpness. For video, where moire on suits and fences is particularly visible, oversampling from a higher resolution sensor than the output resolution helps significantly.
In post-production, Lightroom, Capture One, and Photoshop all offer moire-reduction tools that selectively blur color information in affected areas while preserving luminance detail. Brush-based local correction is usually preferable to a global setting, since the rest of the image rarely needs softening. For stubborn cases, a manual approach (paint a low-saturation layer with mode set to color over the affected region) can clean up patterning where automatic tools fail.
Fashion, product, and architectural photographers shooting on AA-less high-resolution bodies are most likely to encounter moire, and they learn to recognize it on the rear LCD by zooming into suspect areas before leaving the scene. A reshoot with a small angle change is almost always faster than fighting a moire pattern in post.